The observations were carried out with the IRAM 30-m telescope at Pico Veleta (near Granada, Spain) during two observing runs in November 1996 and May 1997. In the first one we observed the six PNe studied by Balser et al. (1997). The observations were made simultaneously in the -1 and 1-0 lines of 12CO. The strongest emitters were then observed in the 13CO lines. In the second run we searched for 13CO -1 and 1-0 emission in 22 objects previously detected in 12CO by Huggins et al. (1996). At the rest frequency of the -1 and 1-0 12CO lines (near 115 and 230 GHz, respectively) the telescope beamsize and efficiency are 24" and 0.6 (115 GHz) and 12" and 0.45 (230 GHz). The smaller beam size at 230 GHz, together with the small scale structure usually found in PNe and with the high intrinsic CO (2-1)/(1-0) line ratio, lead to observed 2-1/1-0 line ratios in the range 2-5 (Bachiller et al. 1993), meaning that the -1 line is much more effective for CO searches. The spectrometers used were two filter banks of 512 1 MHz channels, providing a spectral resolution of 1.3 km s-1 in the 12CO 2-1 line, and 2.6 km s-1 in the 12CO 1-0 line. Typical system temperatures were around 600 K at 115 GHz and 800-1000 K at 230 GHz. The spectra were calibrated with the standard chopper wheel method and are reported here in units of main-beam brightness temperature ().
© European Southern Observatory (ESO) 2000
Online publication: March 17, 2000